1. Field of the Invention
The present invention relates to a method of manufacturing an integrated thin-film solar battery by forming a plurality of unit elements connected to each other on a substrate and an integrated thin-film solar battery manufactured by that method, and more particularly to an improvement in the electrical characteristics of the solar battery by improving the quality of an interface between a semiconductor layer and an electrode layer at a side opposite the light receiving surface, thereby realizing an integrated thin-film solar battery with a high output.
2. Description of the Related Art
The use of solar batteries that convert energy of solar light directly into electrical energy has been now started on a large scale, and crystal-based solar batteries exemplified by single-crystal silicon, polycrystal silicon or the like have already been put into practical use as an outdoor power solar battery. On the other hand, attention has been drawn to a thin-film solar battery such as of amorphous silicon as an inexpensive solar battery because the cost of raw materials used therefor is reduced. However, in general, such a thin-film solar battery is still being developed, and extensive research and development have been conducted in order to develop the thin-film solar battery into a solar battery used outdoors on the basis of the experience of using the thin-film solar battery as a power supply for consumer electronic devices such as calculators.
The thin-film solar battery, as in conventional thin film devices, is manufactured by a process in which the deposition of thin films using CVD sputtering or the like, and patterning of the thin films are repeated so as to obtain a desired structure. There is normally adopted an integrated structure in which a plurality of unit elements are connected in series or parallel on a single substrate. The power solar battery for use outdoors requires a large area substrate that exceeds, for example, 400xc3x97800 (mm).
FIG. 2 is a cross-sectional view showing the structure of the above-mentioned conventional thin-film solar battery. This structure of an integrated thin-film solar battery has been generally used up to now, in which each unit element 15 has a first electrode layer 5, a semiconductor layer 9 made of an amorphous silicon or the like, and a second electrode layer 13, which are stacked one on another in the stated order. The unit elements 15 adjacent to each other are connected in series through a connection opening 7 formed in the semiconductor layer 9. The first electrode layer 5 is usually formed of a transparent electrically conductive film made of, for example, tin oxide (SnO2), zinc oxide ZnO), or indium tin oxide (ITO), and the second electrode layer 13 is formed of a metal film made of, for example, aluminum (Al), silver (Ag), or chromium (Cr).
The above-mentioned conventional integrated thin-film solar battery is manufactured by a method which will be described hereinafter with reference to FIG. 2. On a glass substrate 3, the transparent electrically conductive film made of SnO2, ZnO, or ITO is deposited as the first electrode layer 5, and the first electrode layer 5 is divided into a plurality of sections corresponding to the power generation regions by laser-scribing. Then, a cleaning of the first electrode layer 5 is conducted in order to remove the debris melted and cut off by the laser-scribing. A semiconductor layer 9 made of amorphous silicon with a p-i-n junction structure is deposited on the overall surface of the first electrode layer 5 through a plasma CVD technique. Subsequently, as with the first electrode layer 5, after the semiconductor layer 9 has been divided into a plurality of sections through the laser scribing technique, the semiconductor layer 9 is cleaned in order to remove the debris melted and cut off by the laser-scribing. Each of the plurality of semiconductor layer 9 sections are formed on top of the substrate 3 and the first electrode layer 5 so as to bridge at least two adjacent sections of the plurality of first electrode layer 5 sections. The connector opening 7 (e.g. a VIA hole) is etched in each of the semiconductor layer 9 sections in the vicinity of an adjacent first electrode layer 5 section bridged by the semiconductor layer 9.
Further, as the second electrode layer 13, a metal film made of Al, Ag, Cr or the like is deposited on the semiconductor layer 9 as a single layer or a plurality of layers, and divided into a plurality of sections 13 through the laser scribing technique as with the first electrode layer 5. The second electrode layer 13 contacts the first electrode layer 5 through the connector opening 7 in each of the semiconductor layer 9 sections when filled in with the second electrode layer 13 material, thus completing an integrated thin-film solar battery having a large area.
However, in the above-mentioned conventional integrated thin-film solar battery, it is found that the fill factor (FF value) of its output characteristics is low. Generally, in the manufacture of the integrated thin-film solar battery, the individual cell parameters such as the thickness of the respective electrode layers 5 and 13 or the quality of the semiconductor layer 9 are optimized in order to improve the characteristics. Since the large area of the substrate 3 makes experiments for optimizing the individual process conditions complicated, experimental thin-film solar batteries having a small area are normally manufactured through an easy process as preceding experiments, for evaluating the characteristics obtained thereby. Then, the optimum conditions of the individual processes obtained by the above manner are incorporated the process of manufacturing a thin-film solar battery having a large area.
However, although excellent numerical values may be obtained by the preceding experiments, when the optimum conditions thereof are incorporated in the process of manufacturing a thin-film solar battery having a large area, excellent results as good as the numerical values obtained by the preceding experiment cannot be obtained, and most of the results are lower in FF value. Hence, an improvement in the above-mentioned FF value is indispensable and urgently required for integrated thin-film solar batteries having a large area, in order to improve the conversion efficiency.
Under these circumstances, the present inventors have studied in detail the cause of the lowering of the FF value, with the result that they have proved that the interface between the semiconductor layer 9 and the second electrode layer 13 causes the FF value to be lowered.
FIG. 3 shows a cross-sectional structure of a thin-film solar battery having a small area, used in the above-mentioned preceding experiments. The thin-film solar battery having a small area is obtained in such a manner that a first electrode layer 5 made of SnO2, ZnO, ITO or the like, a semiconductor layer 9 made of an amorphous silicon or the like, and a second electrode layer 13 made of Al, Ag, Cr or the like are stacked on a substrate 3 in the stated order, and patterning of the periphery of the second electrode layer 13 and the semiconductor layer 9 is conducted. The characteristics of the solar battery are then measured by placing a measuring probe on an exposed portion 5a of the first electrode layer 5 and the second electrode layer 13. The thin film solar battery having a small area is not subjected to a cleaning processing since the semiconductor layer 9 is deposited on the substrate 3 before the second electrode layer 13 is deposited thereon, the first electrode layer 5, the semiconductor layer 9, and the second electrode layer 13 being continuously successively formed. In other words, it has been proved that the interface between the semiconductor layer 9 and the second electrode layer 13 absorbs moisture, etc., and the generation of a natural oxide film on the amorphous silicon surface occurs, with the result that the FF value is lowered. However, in the case of the integrated thin-film solar battery having a large area, because the laser scribing technique is applied for patterning, debris melted and cut off by the laser-scribing unavoidably occur. Unless the debris are removed, the adhesion of the second electrode layer 13 and the first electrode layer 5 in the connecting opening 7 is degraded. This adversely affects the characteristics and reliability of the solar battery more than the lowering of the FF value. Hence, the cleaning processing for removing the debris melted and cut off by laser-scribing is essential in the process of manufacturing the integrated thin-film solar battery having a large area.
The above-mentioned conventional method makes it impossible to improve the FF value while conducting the cleaning processing.
The present invention has been made in view of the above circumstances, and therefore an object of the present invention is to provide an integrated thin-film solar battery having a structure which has an improved FF value while still being subjected to the cleaning processing, and a method of manufacturing the same.
In order to solve the above problem, the present invention has been achieved by the provision of an integrated thin-film solar battery having a plurality of unit elements connected in series, in which a plurality of semiconductor layers are disposed on a plurality of first electrode layers which are divided into a plurality of regions on a substrate in such a manner that each of the semiconductor layers is formed on two adjacent first electrodes and has a connection opening on one of the two first electrodes, an electrically conductive layer is formed on each of the semiconductor layers except for the region of the connection opening, a second electrode layer is disposed on each of the electrically conductive layers such that the second electrode layer is electrically connected to one of the two adjacent first electrode layers through the connection opening, to thereby form a region interposed between the second electrode layer and the other first electrode layer as the unit element.
In this example, the electrically conductive layer may be made of a transparent electrically conductive film material mainly containing tin oxide, zinc oxide, or indium tin oxide.
Also, the above-mentioned integrated thin-film solar battery can be manufactured by a method in which, after a plurality of first electrode layers corresponding to a plurality of power generation regions have been formed on a substrate, a plurality of semiconductor layers each having a connection opening that permits a part of each first electrode layer to be exposed therefrom and a plurality of electrically conductive layers are formed on the plurality of semiconductor layers of the plurality of power generation regions, and after a second electrode layer has been formed on the electrically conductive layers, the second electrode layer is divided into a plurality of sections corresponding to the plurality of power generation regions by removing at least the second electrode layer and the electrically conductive layers in the vicinity of each of the connection openings, to thereby form a plurality of unit elements connected in series, each being formed of a region interposed between each first electrode layer and the second electrode layer, on the substrate.
In this example, the electrically conductive layers may be made of a transparent electrically conductive film material mainly containing tin oxide, zinc oxide, or indium tin oxide.
The above-mentioned integrated thin-film solar battery is so designed that the first electrode layer is divided into a plurality of regions on the substrate via the conventional method, the semiconductor layer is formed into a plurality of regions in such a manner that each of the semiconductor layers is formed on two adjacent first electrodes and has a connection opening on one of the two first electrodes, the electrically conductive layer(s) are formed on each of the semiconductor layers except in the region of the connection opening, and the second electrode layer is disposed on each of the electrically conductive layers such that the second electrode layer is electrically connected to one of the two adjacent first electrode layers through the connection opening. With such a structure, there are formed a plurality of unit elements connected in series, each being formed by a region interposed between the second electrode layer and the other first electrode layer.
The thin-film solar battery of the present invention is of a structure in which the connection opening can be defined by the laser scribing technique after the semiconductor layer and the electrically conductive layer(s) are sequentially deposited, since the electrically conductive layer(s) is (are) formed on the semiconductor layer except for the region of the connection opening as described above. That is, the thin-film solar battery of the invention is of a structure such that it can be manufactured without the semiconductor layer being in direct contact with water used for cleaning or external air. Hence, in the integrated thin-film solar battery of the present invention, the natural oxide film derived from the cleaning process is not generated on the surface of the semiconductor layer.
In this example, when the electrically conductive layer is made of a transparent electrically conductive film material such as SnO2, ZnO, or ITO, no natural oxide film is produced on its surface by the cleaning process, and the interface between the electrically conductive layer(s) and the second electrode layer is also maintained in an excellent state. Furthermore, alloying does not occur between the electrically conductive layer and the semiconductor layer. Hence, any factors that lead to the degradation of the FF value can be more completely removed.
The method of manufacturing the thus structured integrated thin-film solar battery is as follows.
First, the first electrode layer is formed on the substrate so as to correspond to the desired plurality of power generation regions. A transparent electrically conductive film made of SnO2, ZnO, ITO or the like is deposited on the substrate, and for integration, the first electrode layer is then melted and segmented in correspondence with the desired plurality of power generation regions via the laser scribing technique. Subsequently, a cleaning process such as water-washing is conducted on the first electrode layer in order to remove the debris melted and cut off by laser-scribing. Although moisture is attracted onto the surface of the first electrode layer through the cleaning process, a natural oxide film is not produced thereon because the first electrode layer is a metal oxide.
Subsequently, the plurality of semiconductor layers each having a connection opening that permits a part of each first electrode layer to be exposed therefrom and the plurality of electrically conductive layers are sequentially formed on the first electrode layers of the plurality of power generation regions by the plasma CVD technique. The semiconductor layer is formed of an amorphous silicon layer having, for example, a p-i-n junction structure, and the electrically conductive layer may be made of SnO2, ZnO, ITO or the like. In the formation of the connection opening, after both the semiconductor layer and the electrically conductive layer have been deposited, they are melted and segmented through the laser scribing technique, to thereby form the connecting opening in a groove shape.
Similarly, a cleaning process such as water-washing is conducted in order to remove the debris melted and cut off by laser-scribing. In this situation, the semiconductor layer is not in direct contact with water because its surface is covered with the electrically conductive layer, with the result that a natural oxide film derived from the cleaning process is not generated on the surface of the semiconductor layer. Although moisture is attracted onto the surface of the electrically conductive layer, a natural oxide film is not produced thereon if the electrically conductive layer is made of a metal oxide such as SnO2, ZnO or ITO. Hence, the electrically conductive layer functions as a protective layer for the semiconductor layer.
Subsequently, after the second electrode layer made of Al, Ag, Cr or the like has been formed on the electrically conductive layers, the second electrode layer is divided in correspondence with the desired plurality of power generation regions by removing at least the second electrode layer and the electrically conductive layer in the vicinity of each of the connection openings, to thereby form a plurality of unit elements connected in series, each being formed of a region interposed between each first electrode layer and the second electrode layer, on the substrate. Similarly, the second electrode layer and the electrically conductive layer are removed through the laser scribing technique, and a cleaning process such as water-washing is conducted in order to remove the debris melted and cut off by laser-scribing.
As described above, in the manufacturing method of the present invention, since the semiconductor layer is not in direct contact with cleaning water or external air, the interface between the semiconductor layer and the second electrode layer is improved in quality, thereby contributing to an improvement in FF value of the solar battery.
It is preferable that a semiconductor layer doped with hydrogen is used as a semiconductor layer contacting with an electrically conductive layer. Because a part of the electrically conductive layer contacting with the semiconductor layer is reduced, with a resulting improvement in electric connection, the FF value is heightened.
It is also preferable that electric resistivity of the electrically conductive layer comprising a transparent metal oxide material is set in a range of 5xc3x9710xe2x88x924 xcexa9xc2x7cm to 4xc3x9710xe2x88x923 xcexa9xc2x7cm to achieve a heightened FF value. When the electrical conductivity is outside the above-described preferred range, the FF value is lowered. In particular, when the electric resistivity of the electrically conductive layers is less than 5xc3x9710xe2x88x924 xcexa9xc2x7cm, ingredients of the electrically conductive layers and the second electrode layers are mutually diffused so that a kind of alloy is generated. As a result, the FF value is lowered. In addition, when the electric resistivity of the electrically conductive layers is more than 4xc3x9710xe2x88x923 xcexa9xc2x7cm, a series resistance of the thin-film solar battery is heightened and the FF value is also consequently lowered.
The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings.